Electromagnetic telemetry systems are used to telemeter information from down in an oil or gas well borehole to surface equipment. A typical telemetry system includes a low frequency transmitter located down in the borehole and a receiver located on the surface. Instead of transmitting the electromagnetic signal over conductors in the borehole, the telemetry system transmits the signal through the earth formations surrounding the borehole.
An electromagnetic telemetry system is useful for acquiring measurement data such as pressure and temperature during fracturing or other production processes. In fracturing, the transmitter is left down in the borehole while pumps pump fluid that contains sand or some other proppant into the borehole. The pumps then pressure the fluid in the borehole to substantial pressures in order to fracture the oil bearing formations. After the formations fracture, the sand fills in the fractures, thereby increasing the permeability of the formation. Conductors conducting the transmitted signal to the surface are unable to withstand the high pressures, temperatures, and the fluids that are used in the fracturing process.
The receiver typically has plural sensors arranged on the surface of the earth in such a manner so as to maximize the reception of the electromagnetic signal. The efficacy of an electromagnetic telemetry system is determined by the signal level and the ambient noise level. Ambient noises includes telluric noise and manmade noise from powerlines and on-site machinery such as pumps and generators. These noise sources can seriously degrade the usefulness of an electromagnetic telemetry system. Thus, it is desirable to reduce the noise in an electromagnetic telemetry system as much as possible.